Acidic bath for electroplating zinc

ABSTRACT

The acidic bath for electroplating zinc comprises an aqueous solution of 200 to 650 g/l of zinc sulfate, 50 to 200 g/l of zinc chloride, 10 to 70 g/l of ammonium chloride, 5 to 30 g/l of sodium succinate, and 0.1 to 10 g/l of nicotinic acid amide and/or 0.01 to 0.1 g/l of ethoxylated- Alpha -naphthol sulphonic acid.

United States Patent [191 Yamagishi et al.

[ Oct. 16, I973 ACIDIC BATH FOR ELECTROPLATING ZINC [75] Inventors: Hidehisa Yamagishi, Yokohama;

Tsutomu Watanabe, Tokyo, both of Japan [73] Assignee: Nippon Kokan Kabushiki Kaisha,

Tokyo, Japan [22] Filed: Oct. 11, 1972 [21] App]. No.: 296,636

[30] Foreign Application Priority Data Nov. 16, 1971 Japan 46/91695 [52] US. Cl 204/55 R [51] Int. Cl G0ln 5/12, GOln 5/46 [58] Field of Search 204/55 R, 43 Z, 44

[56] References Cited UNITED STATES PATENTS 3,669,854 6/1972 Harbulak 204/55 R OTHER PUBLICATIONS Frederick A. Lowenheim, Modern Electroplating, pp. 396-401, (1968).

Primary ExaminerG. L. Kaplan Attorney-Robert D. Flynn et al.

[5 7] ABSTRACT 2 Claims, No Drawings ACIDIC BATH FOR ELECTROPLATING ZINC BACKGROUND OF THE INVENTION This invention relates to an acidic bath for electroplating zinc suitable for forming glossy zinc platings having dense crystal structures under high current densities.

The prior art acidic zinc plating bath utilizes zinc sulfate, zinc chloride or zinc fluoroborate or the like as the source of Zn ions. To enhance the conductivity of the bath, sulfate or chloride is often added to the bath. With such a plating bath, it is possible to obtain platings of considerably high densities when the plating bath is stirred during plating operation. However, under high current densities, the surface of the platings is coarse and is not glossy. Moreover, large crystals are formed. To obviate these difficulties, it has been proposed to add a small quantity of an organic substance such as extract of licorice, glue, gum arabic, dextrose, B naphthol or the like into the bath. Incorporation of such organic substances enables to obtain high quality platings under a certain range of current density, but the practical range of the current density is relatively limited, and it has been impossible to obtain beautiful platings at high current densities, especially in a range of from 50 A/dm to I A/dm SUMMARY OF THE INVENTION It is an object of this invention to provide an improved bath for electroplating zinc which enables to produce glossy and dense plating at such a wide range of current densities as from 10 A/dm to 100 Aldm According to this invention, there is provided an acidic bath for electroplating zinc which comprises a mixture of an aqueous solution containing 200 to 650 g/l of zinc sulfate, 50 to 200 g/l of zinc chloride, 10 to 70 g/l of ammonium chloride, 5 to 30 g/l of sodium succinate and at least one member selected from the group consisting of 0.1 to g/l of nicotinic acid amide and 0.01 to 0.1 g/l of ethoxylated-a-naphthol sulphonic acid.

Zinc sulfate and zinc chloride are used as the source of Zn iOns, and their concentrations are determined on the following basis. More particularly, with a concentration of zinc sulfate of less than 200 g/l it is impossible to use high current densities because the surface of the platings is burnt, whereas with a concentration above 650 g/l, there is a tendency to form precipitates. Said precipitates are zinc ammonium sulfates, which taint a conductor roll for passing an electrical current or precipitate on the surface of plating film to deteriorate the plating film.

With regard to the concentration of zinc chloride, at a concentration of less than 50 g/l, the conductivity is too low to provide high current densities, whereas at a concentration above 200 g/l, the appearance of the platings becomes poor when they are treated with a phosphate or chromic acid.

The purpose of incorporation of ammonium chloride is to improve conductivity. At a concentration of less than 10 g/l, the purpose of ammonium chloride can not be attained whereas at a concentration above 70 g/l, there is a tendency to form precipitates.

When electrolysing using electrolytic bath of this invention, favourable pH of the electrolytic bath is between 3.5 and 4.5 and most desirably pH is about 4.0. pH of the electrolytic bath above 4.5 would cause precipitation of compounds of Fe and Zn. pH of the electrolytic bath below 3.5 would cause generation of hydrogen in the bath, which would be absorbed by the plating film and cause pin holes in the said plating film.

The principal function of sodium succinate is to stabilize the pH of the plating bath. At a concentration of sodium succinate of less than 5 g/l, the action thereof for buffering the pH is not sufficient whereas at a concentration above 30 g/l, the advantage of the sodium succinate is not increased.

Nicotinic acid amide and ethoxylated-a-naphthol sulphonic acid are incorporated for the purpose of providing dense and glossy plated surfaces. They can be used singly or in combination. Incorporation of these compounds forms fine crystals thus providing smooth, glossy and beautiful plated surfaces.

A quantity of 0.1 to 10 g/l is suitable for nicotinic acid amide. A concentration of less than 0.1 g/l is not effective, whereas a concentration of more than 10 g/l does not increase the desired advantage. A suitable concentration of ethoxylated-a-naphthol sulphonic acid is from 0.01 g/] to 0.1 g/l. Thus,a concentration of less than 0.01 g/] is not efficient, whereas a concentration of above 0.1 g/l forms pin holes in the plated surfaces, and blakish plated surfaces of poor appearance;

When a plating bath having a composition in the above described limits is used and when the bath is stirred vigorously during the plating operation it is possible to obtain plated surfaces of a dense crystal structure at a high current density of from 10 A/dm to I00 A/dm DESCRIPTION OF THE PREFERRED EMBODIMENTS To have a better understanding of the invention, following examples are shown but not to be construed as limitation.

EXAMPLE 1 A cold rolled steel plate was plated under following conditions. Composition of the bath:

Nicotinic acid amide (C H N,O) 5 g/l Current density 60 A/dm Plating period 8 sec.

EXAMPLE 3 Composition of the bath:

Zinc sulfate (ZnSO '7H O) 350 gll Zinc chloride (ZnCl,) 200 gll Ammonium chloride (NI-I Cl) 50 g/l Sodium succinate((CH COONa) '6H O) 200 g/l Ethoxylated-a-naphthol sulphonic acid 0.05 gll (CIZHILIOZS) 80 A/dm 4 sec.

Current density Plating period creasing the conductivity of the bath, the improvement wherein said bath comprises an aqueous solution of 200 to 650 g/l of zinc sulfate, 50 to 200 g/l of zinc chloride, 10 to g/l of ammonium chloride, 5 to 30 g/l of sodium succinate and at least one member selected from the group consisting of 0.1 to 10 g/l of nicotinic acid amide and 0.01 to 0.1 g/l of ethoxylated-anaphthol sulphonic acid.

2. A method of electroplating zinc comprising electroplating zinc at a current density of from 10 A/dm to A/dm at a cathode immersed in the bath of claiml. 

2. A method of electroplating zinc comprising electroplating zinc at a current density of from 10 A/dm2 to 100 A/dm2 at a cathode immersed in the bath of claim
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